Light plays a vital role in plant growth and development. Plants perceive light in the environment using a number of photoreceptor systems that control developmental processes such as germination, photomorphogenesis, flowering, and senescence, as well as metabolic processes such as photosynthesis. Plant cells not only obtain energy and chemical disturbance from photons, they also obtain information. The excitation of chlorophyll molecules by visible light provides the energy for CO2 reduction and for maintenance of metabolic activities.
In environments with high primary productivity, the single most important determinant of the light climate experienced by a plant is most often its neighbors. Plants acclimate morphologically and biochemically to the light environment of their neighborhood. They forage for light in the three-dimensional canopy space using a battery of informational photoreceptors. Phytochromes represent a family of red-light-absorbing photoreceptors that can exist in the physiologically inactive Pr form and the active Pfr form. Pr and Pfr are interconvertible by red or far-red (FR) light, respectively. This absorption profile is extremely useful for the detection of shade or the presence of neighboring plants. At high proportions of FR radiation under shade conditions or in dense plant populations, the photoequilibrium is shifted toward the inactive Pr form. Under these conditions, green plants exhibit various symptoms of the shade-avoidance response, such as promotion of stem and petiole elongation, reduced leaf thickness, reduced chlorophyll synthesis, and increased apical dominance. The shade-avoidance response reduces the availability of resources for storage and reproduction.
The ability of plants to adjust their morphology in response to crowding is almost certainly a key element to success for the individual plant in environments of high primary productivity. Increases in yield over the last several decades have been attributed largely to increased density tolerance. Transgenic strategies will be required to further increase the productivity of plants beyond what is possible with conventional breeding. Thus, genes and methods for improving the plant response to density planting are needed.